Development of multidrug resistant bacterial strains has become a global concern. Medicinal plants have been explored as a source of molecules with therapeutic potential for the treatment of various diseases and to the development of better drug against bacterial infections. Thus, the aim of this study was to evaluate the antibacterial activity of the bark and leaves extracts of Anacardium humile (St.) Hil, bark extract of Davilla nitida (Vahl.) kubitski, of leaf extract of Kalanchoe pinnata (Lam.) Pers. and against multidrug resistant bacteria isolated from diabetic foot infections, and also of reference strains from the American Type Culture Collection (ATCC). Antibacterial activity of the bark extract was evaluated by agar Disk-Diffusion (DD), Broth Dilution (BD), “Checkerboard” and “Time-Kill” methods. The bark extract of Davilla nitida (Vahl.) kubistki showed a significant antibacterial activity against all groups of bacteria tested. Broth dilution was more sensitive for determining the antibacterial activity of the bark extract than the Disk-Difusion method. The bark extract inhibited the growth of bacteria with high-levels of antibiotic-resistance, such as Pseudomonas spp. (100,00%), Enterobacer spp. (88,89%), Staphylococcus aureus (54,55%), Streptococcus pneumoniae (75,00%), Staphylococcus saprophyticus (92,86%). The combination of extract with antibiotics resulted in an additive effect against most of the strains tested. “Time-Kill” kinetics profiles of bark extract showed bactericidal and time-dependent properties. Our results suggest the presence of bioactive compounds in this plant that could be useful for the development of new antibacterial agents. The leaf extract of Kalanchoe pinnata (Lam.) Pers. showed antibacterial activity against Citrobacter spp. (100,00%), Citrobacter youngae (ATCC-29935) (100,00%), Enterobacer spp. (33.33%), Pseumodonas spp. (66.67%), Pseudomonas aeruginosa (ATCC-27853) (100.00%). Extract of leaf associated with amoxicillin/clavulanic acid or cefoxitin antibiotics did not show any potentiation of antibacterial activity against both Gram-positive and Gram-negative bacteria, suggesting that extract and antibiotics have independent mechanisms related to inhibition of bacterial growth. Our results showed that the ethanolic leaf extract of Kalanchoe pinnata (Lam.) Pers. had a good antimicrobial activity against some multidrug resistant bacteria. However, further and more specific studies, are recommended to determine the efficacy of this extract in the treatment of bacterial infections. Barks and leaves extracts of Anacardium humile (St.) Hil, both in the Disk-Difusion and Broth Dilution methods, inhibited the growth of Gram-positive and Gram-negative bacteria, as well as the growth of bacteria commonly found in diabetic foot lesions. By the “Checkerboard” method, the combination of bark extract (35,00%) and leaves (40,00%) on Gram-negative bacteria had similar synergistic effects, while for Gram-positive bacteria, leaf extracts (45,00%) presented a synergistic effect more effective than the bark extracts (17,50%). The “Time-Kill” kinetic profile showed bactericidal activity with dose and time-dependent properties, suggesting that bark and leaf extracts may potentiate the effects of antibiotics, suggesting that Anacardium humile (St.) Hil extracts can be used with an alternative source of research for antibacterial agents acting on multidrug resistant Gram- positive and Gram- negative bacteria. However, the isolation of bioactive compounds and additional studies should be performed to understand the mechanisms of bactericidal action, to define real efficacy and toxic effects.
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